I would expand a little on davydd's and Marko's comments about AC running capabilities.
Davydd's is correct that you need to do a calculation about how much power you are going to use a day before deciding what system to get. The hard part is figuring that out when you don't have the van already. Davydd's non AC power consumption is really high compared to many of the rest of us at 6-10 amps, which would translate to as much as 200ah per day, and was substantially higher than his original calculations allowed for. Less fancy systems get easier to get closer, but it still is highly variable. We have not found our compressor frig to run even 50% in the high heat, which surprised us. In Zion last fall at 98* and the frig side of van facing south (our venting is outside, not inside), we still were using around 50ah per day for everything, including 3 fans running all day and DVDs at night, which is pretty typical for us. The frig probably took about 30ah of that so it was only 50% or less of the time, which is also what we thought we were seeing. Bigger frigs use more power, and some brands seem better on power use, and some not as good. If you can get along with the shortcomings of a 3 way frig, your power use drops a whole bunch, if you don't have other big use items.
Anyway you slice it, if the AC uses 100amps, and you can get about 200amps continuous out of the alternator (we are a bit under that with our non standalone version) the engine is going to have to run 1/2 the time. A big battery bank could give you more time between runs, but can't change the ratio. The only exception I can think of that could alter the equation, would be if you were at the van, not having it do it's own thing with Voltstart while you were gone, you could do a little manual trick to change the ratio to hours of AC runtime on batteries for every hour of idling. All you would need to do is shut off the coach AC while the engine is running so the entire 200 amps in the example go to the batteries and they get charged twice a quickly. Turn on the cab AC in the van to keep it cool inside (the cab air is likely at least twice as big as the coach AC). The only issue that might happen would be related to the extra load at idle of both the cab AC and big alternator at full output running at the same time.
Personally, I just don't see the running of AC off batteries as practical unless the runs are short enough, or the batteries and alternator big enough, to be covered by whatever driving you do. Short runs while hiking or at a restaurant or attraction type things. Having the van go on and off all day is just not what we want at our campsite, and I am sure the neighbors wouldn't either, whether allowed or not. I am afraid the whole discussion (not just here, but with the manufacturers also as they try to sell vans) of AC on batteries has been drifting back towards long period off grid AC without a standalone generator, which is kind of where the etrek stuff started, and it didn't appear to be very practical then, and still doesn't, IMO.